The present invention relates to an engine operating with a Stirling cycle wherein the several moving parts are kinematically related to each other so that the engine working gas operates in a cycle consisting of four strokes.
The Stirling cycle engine in its theoretical form consists of four separate strokes which the engine working gas experiences. An expansion stroke allows the working gas to expand at a constant high temperature, during which stroke work is removed from the working gas while heat is added to the working gas from an external source. Following the expansion stroke the working gas is transferred from the environment that maintains the working gas at the constant high temperature to an environment that maintains the working gas at a constant low temperature. This transfer stroke is done at constant expanded working gas volume, and during the transfer stroke heat is removed from the working gas and stored in a device called a regenerator, for use later in the cycle. Theoretically no work is required to perform this transfer stroke since there is no change in the working gas volume. Following the high temperature to low temperature transfer stroke, the working gas volume is forced to contract at the constant low temperature, a stroke which requires heat to be removed from the working gas and work to be added to the working gas. The last stroke to complete the Stirling cycle consists of transferring the working gas from the environment that maintains the working gas at the constant low temperature back to the environment that maintains the working gas at the constant high temperature. This transfer stroke is done at constant contracted working gas volume, and during this transfer stroke the heat which was removed from the working gas during the previous transfer stroke and stored in the regenerator is returned to the working gas, reducing the amount of external heat that must be added to the working gas. Theoretically no work is required to perform this transfer stroke since there is no change in the working gas volume. When taken together the four strokes of the Stirling cycle theoretically equal the Carnot cycle, in which more heat has been added to the working gas than that removed, more work has been removed from the working gas than that added, and the net cycle work equals the work equivalent of the net cycle heat transferred multiplied by the efficiency fraction obtained by dividing the difference of the cycle high temperature and the cycle low temperature by the cycle high absolute temperature.
Hot gas engines designed to operate on the Stirling cycle fail to achieve the Carnot cycle for various reasons. Many of the reasons cannot be eliminated but only reduced, including those involving friction of working gas and engine components, those of heat transfer times being longer than instantaneous, those of heat being transferred in undesired directions, and those of portions of the working gas volume being in spaces such as clearances so the portions cannot be transferred.
A shortcoming that causes prior art hot gas engines to fail to achieve the Carnot cycle, which this invention ameliorates, is that said engines do not allow each of the four above-described strokes to be completed before the next stroke begins, so that simultaneously the working gas is operating in two different strokes. For example, the displacer piston and power piston of a Stirling engine equipped with a rhombic transmission are simultaneously moving at various times in their engine cycle. This allows the working gas volume to be either expanding or contracting while it is also being transferred, which lessens the degree to which the cycle can approach the theoretical Carnot cycle efficiency fraction.